Soft matter keywords

Summary

The purpose of this study was to investigate a different fabrication technique for creating single crystals of pentacene to be used for organic field effect transistors. The liquid-phase crystallization process creates single crystals by dissolving pentacene in solution, then precipitating it as a single crystal. The pentacene crystals formed by this method then had their semiconducting properties measured and these properties were compared to pentacene crystals grown using different methods.

Soft matter discussion

(a) shows TIR images before and after the introduction of <math>Ca^{2+}</math>. (b) and (c) show TEM images of the liposomes before and after the addition of <math>Ca^{2+}</math>

Organic field effect transistors (OFETs) have many favorable properties over current field effect transistors (FETs). The most popular OFET, due to its optimal electronic properties derived from its delocalized pi bond, is pentacene. These properties however, are degraded by the existence of grain boundaries in polycrystalline pentacene as a result of the reduced mobility in charge carriers. Therefore, single crystals of pentacene are primarily used. The most popular way of synthesizing the single crystalline pentacene is currently by physical vapor deposition.

An alternative method of creating pentacene single crystals is by liquid-phase crystallization. In this process pentacene dissolved in trichlorobenzene is purified via various methods. Then the pentacene solution is heated and very slowly cooled so that precipitation occurs as a single crystal. In fact, in this experiment the pentacene was heated to 200C, then cooled at 0.1C per hour. The phase change from solute to solid single crystal is clearly applicable to the soft matter discussion. The image on the left shows the single crystal as seen from AFM. Each step shown is a single molecule step. The entire image has an area of 4umx4um.

The crystalline pentacene was then fabricated onto a thin film of heavily doped silicon oxide to create the OFET. The OFET electrical properties were measured by standard methods. The most important of these properties was the field effect mobility which was measured to be <math>0.4 \frac{cm^2}{Vs}</math>. This number is similar to PVD single crystal pentacene which shows that the fabrication technique was a success. Liquid-phase crystallization is a practical method in producing pentacene single crystals for OFETs.

There are some differences between this method and PVD. For example, there seems to be a large anisotropy in the method used here. That means that the orientation of the crystal, and the direction by which you measure your voltage across makes a difference. This created mobilities that were both higher and lower than expected. Perhaps this can be tuned to create an even better OFET.